U.S. Pat. No. 4,728,981 issued Mar. 1, 1988 to K. C. Koek et al shows an LED printhead assembly including a linear array of LED's on a suitable support and an associated lens structure. The patent discloses a mechanism for mounting the array of LED's and the lens structure with respect to each other and also with respect to a web-type electrophotographic apparatus. That particular mounting structure is used commercially on high-speed, high-volume, nonimpact printers.
In that mounting structure, the printhead assembly is rigidly attached to a holding mechanism for the bearings of a roller supporting a photoconductive web. That rigid attachment uses mounting pins which permit removal of the printhead assembly and reinsertion of it without additional adjustments. Because of the rigid relationship between the printhead assembly and the bearings for the roller, focus is readily maintained on the photoconductive web.
This structure is suitable for such high volume applications where the exact location of the bearings of the roller relative to certain locating structure associated with the bearings is readily maintainable. However, there are other applications in which the exact location of the roller is not readily maintained. For example, it is much less costly to provide a support for a roller that tolerates some misalignment or inaccuracy in location. Similarly, the roller can be part of a support, commonly called a "film core" which is different from the frame of the apparatus to which the printhead assembly is fixed. If the core is removable to replace the belt, it may be prohibitively costly to assure that the core replacement process exactly positions the roller with respect to a rigidly fixed printhead assembly.
U.S. Pat. No. 4,703,334, granted Oct. 27, 1987 to Mochimaru et al, shows a structure in which a printhead assembly is rigidly fixed to the frame of an apparatus and a photoreceptive web is urged against the printhead assembly by a resiliently urged back-up roller that contacts both the web and key printhead assembly contact surfaces. This structure corrects minor variations in locations of the belt (or a drum) but has relatively few degrees of freedom.